The present invention relates to a radar sensor having a transceiver device, an analog/digital converter for converting the received signals into digital data, a memory device for storing the digital data as a useful data set which provides the digital data for one recording period as a function of time, a transformation device for calculating one spectrum of the useful data set, and an analyzer for analyzing the spectrum.
In particular, the present invention deals with a radar sensor which forms a component of a driver assist system in a motor vehicle, e.g., an ACC system (adaptive cruise control) for an automatic radar-supported distance regulation.
A typical example for a radar sensor of the above-mentioned type is an FMCW radar (frequency modulated continuous wave) in which the frequency of the transmitted radar signal is periodically modulated at a certain ramp gradient. The frequency of a signal which was reflected from a radar target and is subsequently received by the radar antenna at a certain point in time thus differs from the frequency of the signal transmitted at this point in time by an amount which is a function of the signal propagation delay and thus of the distance to the radar target, and also of the Doppler shift and thus of the relative velocity of the radar target. The received signal is mixed with the signal transmitted at this point in time in a mixer in the radar sensor, so that a low-frequency signal whose frequency corresponds to the frequency difference between the transmitted signal and the received signal is obtained by pulsation. This low-frequency signal is then digitized in the analog/digital converter using a suitable time resolution. The digitized data is recorded over a certain recording period which, for example, corresponds to the length of the ramp with which the transmitted signal is modulated. Via an algorithm, which is known as “fast Fourier transformation” (FFT), a spectrum is formed from the data set obtained in this way. In this spectrum, each detected radar target is represented by a peak which rises more or less clearly above the noise level. By repeating this procedure using different ramp gradients it is possible to eliminate the ambiguity between the propagation time-dependent frequency shift and the Doppler shift so that the distance to and the relative velocity of the radar target may be calculated.
An angle-resolving radar sensor which generates multiple radar lobes slightly offset toward one another is mostly used in motor vehicles, and the above-described signal processing and signal analysis take place separately for each radar lobe, preferably in parallel channels.
In order to be able to follow the located radar targets with high time resolution, digital data processing should take place at a bit rate as high as possible. However, the occasional occurrence of errors may not be completely ruled out. Such errors may not only occur due to the fact that the received signal is noisy, but also due to the fact that electromagnetic interference signals interfere with the analyzing electronics or that individual data bits are corrupted during data transmission and data processing (bit aliasing). Further error sources are software errors or hardware errors which are detectable only sporadically under unfavorable circumstances and have therefore been overlooked during quality control.
For reasons of traffic safety, the radar sensor should make it possible to locate other vehicles and obstacles as reliably as possible. Moreover, efforts are made to upgrade the functionality of driver assist systems to the point of completely autonomous vehicle control as a long-term objective. To the extent to which new and increasingly complex tasks are assigned to the driver assist system, demands on the reliability of the radar sensor also increase.